Dual modulation displays, such as may be used in high brightness and/or high-dynamic range (HDR) displays, for example, may incorporate a spatially modulated light source such as those described in PCT Patent Application Publication Nos. WO02/069030, WO03/077013, WO2006/010244 and WO2008/092276. Such displays comprise a light source modulation layer (e.g. a spatially modulated backlight) and a display modulation layer. The light source modulation layer may be driven to produce a comparatively low-resolution representation of an image which is subsequently provided to the display modulation layer. The low-resolution representation is further modulated by the display modulation layer to provide a higher resolution image which is viewed by the observer. The light source modulation layer may comprise a matrix of actively modulated light sources, such as light-emitting diodes (LEDs), for example. The display modulation layer, which may be positioned and/or aligned to receive light from the light source modulation layer, may comprise a liquid crystal display (LCD), for example. The brightness of a pixel on the display modulation layer is therefore affected by the variable localized brightness across the light source modulation layer.
Some artifacts may be apparent in an image displayed on a dual modulation display if the light source modulation layer provides too much light or insufficient light to areas of the display modulation layer which cannot be entirely compensated for by driving the pixels on the display modulation layer to minimum or maximum light transmissivity states. For example:                If the light source modulation layer provides too much light to a certain area of the display modulation layer, the affected area of the display modulation layer may appear to be clipped to a certain minimum local brightness level, even if the pixels in the affected area are driven to a state of minimum transmissivity (i.e. a fully “closed” position). This may result in a loss of detail in the affected area, raised black levels, and color shifts.        If the light source modulation layer does not provide enough light to a certain area of the display modulation layer, the affected area of the display modulation layer may appear to be clipped to a certain local maximum brightness level, even if the pixels in the affected area are driven to a state of maximum transmissivity (i.e. a fully “open” position). This may result in a loss of appearance of texture in the affected area, decreased white levels, and color shifts.        
There is a desire for methods and apparatus for driving dual modulation displays to reduce the appearance of these artifacts.
For efficiency purposes it is desirable to drive the light source modulation layer to provide the minimum amount of light to the display modulation layer to achieve the desired image brightness. Dual modulation displays may be running at less than optimal efficiency if the pixels of the display modulation layer are driven to lower transmissivity states (e.g. near or in a “closed” position) in order to compensate for overly bright regions of the light source modulation layer. There is a desire for methods and apparatus for driving dual modulation displays in a more efficient manner.